Non-destructive testing apparatus

ABSTRACT

Apparatus for non-destructively testing material to enable detection of any damage sites, comprising means for generating localised heating at any damage site in the material, and means for imaging the material to enable detection of any localised heating at the damage site.

[0001] The invention relates to non-destructive testing apparatus andtechniques and in particular to thermal non-destructive testing in orderto detect barely visible impact damage (BVID). The invention isparticularly, but not exclusively, directed to detecting such damage incompound materials as used on aircraft wing skins.

[0002] An object of the invention is to provide improvements in suchapparatus and in particular to provide a relatively inexpensive, easy tooperate system for non-destructive testing of materials. It is aparticular object of the invention to provide a system which islocatable in position on a material in use, enabling relatively simpletesting of material, thereby requiring minimal down time, for example ofan aircraft being tested for barely visible impact damage on itscomposite wing skin structure.

[0003] One aspect of the invention provides apparatus fornon-destructively testing material to enable detection of any damagesite, comprising means for generating localised heating at any damagesite and means for imaging the material to enable detection of anylocalised heating at a damage site.

[0004] Preferably the heating means comprises a piezo-electric actuator,and/or the actuator can be adapted to be bonded to the material orembedded within the material in use. Preferably a signal generator isprovided operable to drive the piezo-electric actuator in use.

[0005] The piezo-electric actuator preferably comprises two regions ofpiezo-electric material. The two regions can comprise a stack of twopieces of piezo-electric wafers. The piezo-electric actuator canoperably be driven in an extension mode, thereby to impact vibrations tothe material in use. Preferably an array of piezo-electric actuators ismounted is provided which are adapted to be mounted on a material.

[0006] A signal generator which operatively drives the heating means isalso preferably provided. The signal generator, preferably operablygenerates a frequency modulated signal. The frequency modulated signalcan comprise a sine wave signal. The frequency modulated signal cancomprise a phase-continuous frequency swept signal. Preferably themodulation is substantially linear. Also the carrier frequency ispreferably in the range of about 1 to about 2000 Hz.

[0007] Preferably the carrier frequency is in the range of about 300 to900 Hz, and/or in the range of about 550 to about 750 Hz. Preferably themodulation frequency of the carrier frequency range is between about0.01 Hz and 1 Hz, and/or between about 0.1 and 0.3 Hz, and/or about 0.2Hz.

[0008] Another aspect of the invention provides an array ofpiezo-electric actuators adapted to be mounted on a material to enablelocalised heating within the material at any damage sites. A furtheraspect of the invention provides material for use in sufficientlycritical situations where damage sites in the material are required tobe detected, which material comprises one or more piezo-electricactuators bonded to or embedded in the material operably to enabledetection of any damage sites within the material. A further aspect ofthe invention provides apparatus for non-destructive testing material toenable detection of any damage sites in a substrate of material,comprising a signal generator which operably generates a frequencymodulated signal for driving a piezo-electric actuator and means fordetecting localised heating at a damage site, preferably comprising athermal camera and controller such as a personal computer. Preferably anoutput from the signal generator is adapted to be connected to one ormore piezoelectric actuators.

[0009] A further aspect of the invention provides apparatus fordetecting impact damage comprising a piezo-electric actuator mounted ona substrate such as an aircraft wing skin, and a signal analyseroperably connected to the piezoelectric actuator for detecting anacoustic signal indicative of impact on the substrate. Preferably anarray of piezo-electric actuators is provided.

[0010] A further aspect of the invention provides impact damagedetection apparatus comprising apparatus according to the first aspectof the invention and the last aspect.

[0011] Also an aspect of the invention provides an array of two or morepiezo-electric actuators operably in communication with a connectorenabling communication with real time impact damage detection apparatus.

[0012] An embodiment of the invention will now be described, by way ofexample only, with reference to the accompanying drawings, in which:

[0013]FIG. 1 is a schematic block diagram of apparatus according to theinvention,

[0014]FIG. 2 is a plan view of a piezo-electric actuator forming part ofthe apparatus shown in FIG. 1,

[0015]FIG. 3 is an exclamatic perspective view of the actuator shown inFIG. 2,

[0016]FIG. 4 is an exclamatic diagram of part of the signal used todrive the actuator,

[0017]FIG. 5 an introduction of an image captured using the apparatusshown in FIG. 1,

[0018]FIG. 6 is a graph of the signal ampitute along line one shown inthe image in FIG. 5.

[0019]FIG. 7 is a schematic block diagram of a second embodiment ofapparatus according to the invention, and

[0020]FIG. 8 is a schematic graph illustrating real time detection ofimpact damage during a flight using the apparatus shown in FIG. 7.

[0021] Referring to FIG. 1 there is shown a non-destructive testingapparatus 10 according to the invention which comprises a computer 12,means 14 for causing localised heating within a substrate S and means 16for imaging the substrate.

[0022] The heating means 14 can comprise a function generator 18operably in communication with the computer 12. The function generator18 drives an amplifier 20 which in turn generates a signal topiezo-electric actuator 22 attached to substrate S.

[0023] The function generator 18 oprerably produces a frequencymodulated signal of the type described later in relation to FIG. 4. Theoutput from function generator 18 is fed to piezo-electric amplifier 20which amplifies the frequency modulated signal and drives piezo-electricactuator 22. Preferably the amplifier is capable of generating a signalin the order of 40 watts, such as a 200 (rms) volt, 0.2 amp signal.

[0024] The piezo-electric actuator 22 is shown in more detail in FIGS. 2and 3. The specific actuator shown is one available from Active ControlExperts of Cambridge, Mass., USA (ACX) and particularly their actuatorQP20N. The particular device is a twin strip actuator comprising a stackof two piezo-electric wafers capable of a full scale strain extension inthe order of 0.000264, and having a capacitance in the order of 0.12micro Farads, and full scale voltage range of ±200 volts. The twin layerstructure is shown in the schematic exploded shown in FIG. 3 where twopiezo-electric layers 26 and 28 are shown in a stacked arrangement. Theactuator 22 further comprises an input 24 for connection to amplifier20, and 4 electrodes 30, only the upper electrode of which is shown inFIG. 3. The actuator 22 can be moulded into or mounted onto a substrateS for example using an epoxy resin adhesive.

[0025] Referring back to FIG. 1, the imaging means 16 preferablycomprises a thermal camera 32, connected to the computer 12. Camera 32is preferably a thermal analysis camera capable of synchronised lock-inthermography. A suitable thermal camera 32 is available from CEDIP ofCroissy-Beaubourg, France for example which comprises of a 128×128 focalplane array and stirling engine cooler.

[0026] The apparatus 10 is driven by computer 12 which can be a personalcomputer having user inputs such as a keyboard and mouse anduser-display such as a video monitor, as well as input and outputconnections enabling synchronisation of actuation of the heating means14 and imaging means 16, via communication lines such as appropriatewiring.

[0027] In use, computer 12 is used to synchronise operation of thepiezo-electric actuator 22 and thermal camera 32 in order to optimisedata capture by the camera. The data is stored and processed usingcomputer 12 to enable analysis of images of the sample, thereby toenable detection of impact damage D within a sample. The system isadapted to enable detection of barely visible impact damage such asdamage created by a six Joule impact on a composite structure such as anaircraft wing structure, forming substrate S.

[0028] In one preferred form, function generator 18 generates afrequency modulated signal 34 as shown in FIG. 4 which is preferably aphase continuous, linearly ramped signal as shown. The signal generatedhas a lower frequency SL and an upper frequency SU. The frequency ismodulated substantially linearly, albeit incrementally such as in 1024quantifiable steps, depending on the nature of the function generator18, over a time period T. The carrier frequency of signal 34, is halfthe sum of the lower frequency and upper frequency, (FL+FU)/2, whereasthe modulation frequency is the inverse of the time period (1/T.)

[0029] Preferably the appropriately amplified signal 34 is applied topiezo-electric actuator 22 by amplifier 20 over a predetermined periodfor data to be captured by camera 32 to enable detection of any damagesites such as barely visible impact damage D within a substrate S. Inone form, a carrier frequency of 200 Hz is used with a modulationfrequency in the order of 0.1 Hz and with an upper frequency in theorder of 995 Hz.

[0030] Data capture by thermal camera 32 is preferably at a rate of 25Hz for one complete image. Preferably 2500 images are captured andpreferably 50 background images of the substrate prior to heating areused to enable background subtraction from the captured data images. Inthis example of the preferred embodiment the thermal image illustratedin FIG. 5 is, therefore, captured in 100 seconds. Preferably each imagecomprises 128×128 pixels according to the specification of thermalcamera 32.

[0031] Referring to FIG. 5, there is shown an example of an output imageas viewed on computer 12 from substrate S. FIG. 5 shows a lighter regionat the top which represents the piezo-electric actuator 22 and has alighter dot at its centre which is the position of damage site D shownFIG. 1. A horizontally line 1 is shown through the damage site. Theindividual data, such as digital level within the image, along line 1 isshown graphically in FIG. 6 where the signal is at a maximum at thedamage site D. This represents the area of localised heating at thedamage site D as observed by thermal camera 32.

[0032] Accordingly, the apparatus 10 enables qualitative analysis by auser to detect a damage site D by viewing results of images at thecomputer 12, and more accurately statistical analysis of data generatedby the camera 32 in analysing data to detect a peak signal such as shownin FIG. 6.

[0033] The invention envisages composite structures such as aircraftwings, helicopter blades and so on having one or more piezo-electricactuators mounted or embedded therein, such that the actuator is capableof being coupled to apparatus 10 according to the invention.Beneficially, such an array of one or more piezo-electric actuators canbe coupled to an on-board control system adapted to drive one or moreactuators within the array as appropriate for the imaging means toenable detection of impact damage within the material. Beneficially,such an array of actuators coupled to a controller can provide adetection mechanism for monitoring impact damage on a real-time basis.For example, the actuators can be used as sensors to determine theexistence of vibrations in an aircraft wing, thereby to detect largeimpact due to an electric signal generated by a piezo-electric actuatorduring use of an aircraft.

[0034] Referring to FIGS. 7 and 8, there is shown apparatus 100 whichenables no destructive testing using apparatus 102 and separated by thedotted line L in FIG. 7. Apparatus 102 comprises substantial all thosefeatures shown in FIG. 1 where like components are given the same twodigits reference prefix with the numeral 1. Accordingly, apparatus 1 and2 comprises a computer 112, function generator 118, amplifier 120, and athermal camera 132. Apparatus 102 further comprises a user interface 146such as display, warning signals or similar devices.

[0035] Apparatus 100 further comprises an impact detector, apparatus 104which can be integrated with apparatus 102 or independent thereof. Bothapparatuses 102 and 104 are connectable to apparatus 106 comprises aconnector 140 and an array of piezo-electric actuator 122 mounted in orattached to a substrate S such as an aircraft wing skin. Accordingly, anaircraft can comprise apparatus 106 wherein a wing skin has an array ofpiezo-electric actuators 122 mounted thereon which all communicate withconnector 140 suitable located on the aircraft such as on its undersideenabling connection to non-destructive testing apparatus 102 when theaircraft is on the ground or in a hanger for example, thereby to enablenon-destructive testing using the technique describe in relation toapparatus 10.

[0036] Similarly, connector 140 is preferably suitably located forconnection to the impact detector apparatus 104 which preferably enablesreal time collection of data and comprises a signal analyser 142comprising for Example a suitable receiver, amplifier and microprocessorfor analysing a signal from each of the piezo-electric actuators 122.The type of signal observed at analyser 142 is shown in FIG. 8. Signal150 comprises an initial base line having a substantially equalamplitude over time. When an impact occurs near a piezo-electricactuator 122 this causes vibration at the piezo-electric actuatorthereby to generate an electrical signal which is communicated toanalyser 142 via connector 140. The signal comprises a pulse 154 over atime period t2 minus t1. The pulse 154 is seen as an increasing signalamplitude from a piezo-electric actuator 122 which passes through afirst amplitude threshold level TH1 and subsequent second amplitudelevel TH2 before returning to a new baseline amplitude 152 betweenthreshold TH1 and TH2. Accordingly, before the impact event at time t1the piezo-electric actuator records acoustic emission below thresholdlevel TH1. At time t1 the impact event occurs producing signal amplitudeabove threshold level TH2 which ends at time t2, after which a largerbackground threshold level 152 is recorded due to the continuousmovement of impact damage material. By setting detection the thresholdlevels at the analyser 142 appropriately, an impact damage warning cantherefore be giving by apparatus 104, for example in the aircraft cabincockpit via a user interface 144 such as a warning light, or at amaintenance hanger, via a radio transmitter 144 to highlight potentialproblems with the structure so that repair crews can be ready when theaircraft lands. Analyser 142 can comprise memory for data from a flightwhich is communicable with computer 112 of apparatus 100 thereby toprovide an overall apparatus which is capable of real time impact damageanalysis and post impact damage analysis whereby computer 112 isconfigured to scrutinise more vigorously Sensibly those areas ofsubstrate S local to an actuator 112 from which any actual or potentialimpact signals 154 have been observed.

1. Apparatus for non-destructively testing material to enable detectionof any damage sites, comprising means for generating localised heatingat any damage site in the material, and means for imaging the materialto enable detection of any localised heating at the damage site. 2.Apparatus according to claim 1 where in the heating means comprises apiezo-electric actuator.
 3. Apparatus according to claim 2, wherein theactuator is adapted to be bonded to the material or embedded within thematerial in use.
 4. Apparatus according to claim 2 or 3 comprising asignal generator operable to drive the piezoelectric actuator in use. 5.Apparatus according to claim 4 where 2, 3 or 4 were in thepiezo-electric actuator comprises two regions of piezo-electricmaterial.
 6. Apparatus according to claim 5 wherein the two regionscomprise a stack of two pieces of piezo-electric wafers.
 7. Apparatusaccording to any claims 2 to 6 where in the piezo-electric actuator isoperably driven in an extension mode, thereby to impart vibrations tothe material in use.
 8. Apparatus according to any claims 2 to 7comprising an array of piezo-electric actuators adapted to be mounted ona material.
 9. Apparatus according to any preceding claims comprising asignal generator which operatively drives the heating means. 10.Apparatus according to claim 9 wherein the signal generator, operablygenerates a frequency modulated signal.
 11. Apparatus according to claim10 wherein the frequency modulated signal comprises a sine wave signal.12. Apparatus according to claims 10 or 11 wherein the frequencymodulated signal comprises a phase-continuous frequency swept signal.13. Apparatus according to any of the claims 10, 11 or 12 wherein themodulation is substantially linear.
 14. Apparatus according to any ofclaims 10 to 13 wherein the carrier frequency is in the range of about 1to about 2000 Hz.
 15. Apparatus according to any of claims 10 to 14wherein the carrier frequency is in the range of about 300 to about 900Hz.
 16. Apparatus according to any claims 10 to 15 wherein the carrierfrequency is in the range of about 550 to about 750 Hz.
 17. Apparatusaccording to any claims 10 to 16 wherein the modulation frequency of thecarrier frequency range is between about 0.01 Hz and about 1 Hz. 18.Apparatus according to claim 17 wherein the modulation frequency isbetween about 0.1 and 0.3 Hz.
 19. Apparatus according to claim 18wherein the modulation frequency is about 0.2 Hz.
 20. An array ofpiezo-electric actuators adapted to be mounted on a material to enablelocalised heating within the material at any damage sites.
 21. Amaterial for use in sufficiently critical situations where damage sitesin the material are required to be detected, which material comprisesone or more piezo-electric actuators bonded to or embedded in thematerial operably to enable detection of any damage sites within thematerial.
 22. Apparatus for non-destructive testing materials to enabledetection of any damage sites in a substrate of material, comprising asignal generator which operably generates a frequency modulated signalfor driving a piezo-electric actuator and means for detecting localisedheating at a damaged site, preferably comprising a thermal camera andcontroller such as a personal computer.
 23. Apparatus according to claim22 comprising an output from the signal generator adapted to beconnected to one or more piezo-electric actuators.
 24. Apparatus fordetecting impact damage comprising a piezo-electric actuator mounted ona substrate such as an aircraft wing skin, and a signal analyseroperably connected to the piezo-electric actuator for detecting anacoustic signal indicative of impact on the substrate.
 25. Apparatusaccording claim 24 comprising an array of piezo-electric actuators. 26.Impact damage detection apparatus comprising apparatus according toclaim 1 and any claim dependent thereon, and claim 24 and any claimdependent thereon.
 27. An array of two or more piezo-electric actuatorsoperably in communication with a connector enabling communication withreal time impact damage detection apparatus.